Add new column or additional table to join two existed tables?

I need to add a new "clmX" column to an existing "tb1" table, which would be a foreign key to another "tb2" table, but some rows will have a null value in the "clmX" column.

Which option is better if we are thinking about designing a database:

a) – add a new column "clmX" to an existing table (as i described above) or

b) – add a new table "tb3", which contains two columns: id_t1, id_tb2 and serves as a connection table between "tb1" and "tb2"?

>Solution :

There is no need to create another table unless it is a many-to-many relation. ie:

CREATE TABLE t1 (id INT IDENTITY PRIMARY KEY NOT NULL, dummy VARCHAR(10));
INSERT dbo.t1(dummy)VALUES('a'), ('b'), ('c'), ('d'), ('e');
CREATE TABLE t2 (id INT IDENTITY PRIMARY KEY NOT NULL, [name] VARCHAR(100) NOT NULL);
INSERT INTO dbo.t2([name])VALUES('c1'), ('c2'), ('c3'), ('c4');
ALTER TABLE t1
ADD clmX INT CONSTRAINT t1_t2 FOREIGN KEY(clmX)REFERENCES t2(id)ON UPDATE CASCADE ON DELETE SET NULL;
UPDATE t1 SET clmX=1 WHERE id=1;
UPDATE t1 SET clmX=2 WHERE id=2;
UPDATE t1 SET clmX=3 WHERE id=3;
SELECT * FROM t1;
SELECT * FROM t2;

fiddle